diff --git a/approvaltests/src/main/java/org/approvaltests/reporters/GenericDiffReporter.java b/approvaltests/src/main/java/org/approvaltests/reporters/GenericDiffReporter.java
index a725b669..14634063 100644
--- a/approvaltests/src/main/java/org/approvaltests/reporters/GenericDiffReporter.java
+++ b/approvaltests/src/main/java/org/approvaltests/reporters/GenericDiffReporter.java
@@ -47,7 +47,7 @@ public GenericDiffReporter(String diffProgram, String argumentsFormat, String di
     this.diffProgram = diffProgram;
     this.arguments = argumentsFormat;
     this.diffProgramNotFoundMessage = diffProgramNotFoundMessage;
-    validExtensions = validFileExtensions;
+    this.validExtensions = validFileExtensions;
   }
 
   public GenericDiffReporter(DiffInfo info)